Evidence that a transcortical pathway contributes to stretch reflexes in the tibialis anterior muscle in man

Differential modulation of spinal and corticospinal excitability during drop jumps

Previously it was shown that spinal excitability during hopping and drop jumping is high in the initial phase of ground contact when the muscle is stretched but decreases toward takeoff. To further understand motor control of stretch-shortening cycle, this study aimed to compare modulation of spinal and corticospinal excitability at distinct phases following ground contact in drop jump. Motor-evoked potentials (MEPs) induced by transcranial magnetic stimulation (TMS) and H-reflexes were elicited at the time of the short (SLR)-, medium (MLR)-, and long (LLR, LLR(2))-latency responses of the soleus muscle (SOL) after jumps from 31 cm height. MEPs and H-reflexes were expressed relative to the background electromyographic (EMG) activity. H-reflexes were highly facilitated at SLR (172%) and then progressively decreased (MLR = 133%; LLR = 123%; LLR(2) = 110%). TMS showed no effect at SLR, MLR, and LLR, whereas MEPs were significantly facilitated at the LLR(2) (122%; P = 0.003). Background EMG was highest at LLR and lowest at LLR(2). Strong H-reflex facilitation at the beginning of the stance phase indicated significant contribution of Iotaa-afferent input to the alpha-motoneurons during this phase that then progressively declined toward takeoff. Conversely, corticospinal excitability was exclusively increased at the phase of push off (LLR(2), approximately 120 ms). It is argued that corticomotoneurons increased their excitability at LLR(2). At LLR ( approximately 90 ms), Iotaa-afferent transmission as well as corticospinal excitability was low, whereas background EMG was high. Therefore it is speculated that other sources, presumably subcortical in origin, contributed to the EMG activity at LLR in drop jumps.

Role of Sustained Excitability of the Leg Motor Cortex After Transcranial Magnetic Stimulation in Associative Plasticity

Changes in the strength of corticospinal projections to muscles in the upper and lower limbs are induced in conscious humans after paired associative stimulation (PAS) to the motor cortex. We tested whether an intervention of PAS consisting of 90 low-frequency (0.1-Hz) stimuli to the common peroneal nerve combined with suprathreshold transcranial magnetic stimulation (TMS) produces specific changes to the motor-evoked potentials (MEPs) in lower leg muscles if the afferent volley from peripheral stimulation is timed to arrive at the motor cortex after TMS-induced firing of corticospinal neurons. Unlike PAS in the hand, MEP facilitation in the leg was produced when sensory inputs were estimated to arrive at the motor cortex over a range of 15 to 90 ms after cortical stimulation. We examined whether this broad range of facilitation occurred as a result of prolonged subthreshold excitability of the motor cortex after a single pulse of suprathreshold TMS so that coincident excitation from sensory inputs arriving many milliseconds after TMS can occur. We found that significant facilitation of MEP responses (>200%) occurred when the motor cortex was conditioned with suprathreshold TMS tens of milliseconds earlier. Likewise, it was possible to induce strong MEP facilitation (85% at 60 min) when afferent inputs were directly paired with subthreshold TMS. We argue that in the leg motor cortex, facilitation of MEP responses from PAS occurred over a large range of interstimulus intervals as a result of the paired activation of sensory inputs with sustained, subthreshold activity of cortical neurons that follow a pulse of suprathreshold TMS.

Voluntary modulation of human stretch reflexes

It has been postulated that the central nervous system (CNS) can tune the mechanical behavior of a joint by altering reflex stiffness in a task-dependant manner. However, most of the evidence supporting this hypothesis has come from the analysis of H-reflexes or electromyogram (EMG) responses. Changes in overall stiffness have been documented but, as yet, there is no direct evidence that the CNS can control reflex stiffness independently of the intrinsic stiffness. We have used a novel identification algorithm to estimate intrinsic and reflex stiffness and feed it back to subjects in real-time. Using this biofeedback, subjects could learn to control reflex stiffness independently of intrinsic stiffness. At low torque levels, subjects could vary their reflex stiffness gain by a factor of 4, while maintaining elastic stiffness and torque constant. EMG measurements confirmed that the contraction levels of the ankle muscles remained constant. Further experiments showed that subjects could change their reflexes rapidly on command. Thus, we conclude that the CNS can control reflex stiffness independently and so has great flexibility in adjusting the mechanical properties of a joint to meet functional requirements.

Cortical and spinal adaptations induced by balance training: correlation between stance stability and corticospinal activation

AIM

To determine the sites of adaptation responsible for improved stance stability after balance (=sensorimotor) training, changes in corticospinal and spinal excitability were investigated in 23 healthy subjects.

METHODS

Neural adaptations were assessed by means of H-reflex stimulation, transcranial magnetic stimulation (TMS) and conditioning of the H-reflex by TMS (Hcond) before and after 4 weeks of balance training. All measurements were performed during stance perturbation on a treadmill. Fast posterior translations induced short- (SLR), medium- and long-latency responses (LLR) in the soleus muscle. Motor-evoked potential- (MEP) and Hcond-amplitudes as well as Hmax/Mmax ratios were determined at SLR and LLR. Postural stability was measured during perturbation on the treadmill.

RESULTS

Balance training improved postural stability. Hmax/Mmax ratios were significantly decreased at LLR. MEPs and Hcond revealed significantly reduced facilitation at LLR following training. A negative correlation between adaptations of Hcond and changes in stance stability was observed (r = -0.87; P < 0.01) while no correlation was found between stance stability and changes in Hmax/Mmax ratio. No changes in any parameter occurred at the spinally organized SLR and in the control group.

CONCLUSION

The decrease in MEP- and Hcond-facilitation implies reduced corticospinal and cortical excitability at the transcortically mediated LLR. Changes in cortical excitability were directly related to improvements in stance stability as shown by correlation of these parameters. The absence of such a correlation between Hmax/Mmax ratios and stance stability suggests that mainly supraspinal adaptations contributed to improved balance performance following training.

Muscle reflexes and synergies triggered by an unexpected support surface height during walking

An important phase in the step cycle is foot contact. When the moment of foot contact differs from the one expected, a fast response is needed. Such a mismatch can be caused by hitting a support surface earlier or later than expected. To study this, experiments were performed with healthy young adults who walked on a platform that was unexpectedly at a lowered (5 cm) or at a level height. Glasses blocked the lower visual field. In the unexpectedly lowered trials, the absence of expected heel contact triggered responses in the ipsilateral anti-gravity muscles [ipsilateral medial gastrocnemius (MGi), ipsilateral rectus femoris (RFi)] and contralateral flexor muscles [contralateral tibialis anterior (TAc), contralaterial biceps femoris (BFc)] with latencies of 47-69 ms. After the delayed heel contact, enhanced activity was found in the MGi, RFi, and TAc muscles. This specific muscle synergy was presumably activated to arrest the forward propulsion of the body. In contrast, when the surface was unexpectedly at level height, the subjects expected to step down, and the leg briefly yielded. A muscle synergy was activated at 46-81 ms that flexed the ipsilateral knee (TAi, BFi, RFi) and extended the contralateral one (MGc, BFc) to unload the perturbed leg and delay the contralateral swing phase. Both conditions triggered a fast functionally relevant muscle synergy because of a mismatch between the expected and actual sensory feedback at the moment of foot contact. The results are consistent with an internal model that compares the expected with the actual sensory feedback. The short latency of the response suggests a subcortical, possibly cerebellar pathway.

Changes in Excitability of the Cortical Projections to the Human Tibialis Anterior After Paired Associative Stimulation

Paired associative stimulation (PAS) based on Hebb's law of association can induce plastic changes in the intact human. The optimal interstimulus interval (ISI) between the peripheral nerve and transcranial magnetic stimulus is not known for muscles of the lower leg. The aims of this study were to investigate the effect of PAS for a variety of ISIs and to explore the efficacy of PAS when applied during dynamic activation of the target muscle. PAS was applied at 0.2 Hz for 30 min with the tibialis anterior (TA) at rest. The ISI was varied randomly in seven sessions ( n = 5). Subsequently, PAS was applied ( n = 14, ISI = 55 ms) with the TA relaxed or dorsi-flexing. Finally, an optimized ISI based on the subject somatosensory evoked potential (SEP) latency plus a central processing delay (6 ms) was used ( n = 13). Motor-evoked potentials (MEPs) were elicited in the TA before and after the intervention, and the size of the TA MEP was extracted. ISIs of 45, 50, and 55 ms increased and 40 ms decreased TA MEP significantly ( P = 0.01). PAS during dorsi-flexion increased TA MEP size by 92% ( P = 0.001). PAS delivered at rest resulted in a nonsignificant increase; however, when the ISI was optimized from SEP latency recordings, all subjects showed significant increases ( P = 0.002). No changes in MEP size occurred in the antagonist. Results confirm that the excitability of the corticospinal projections to the TA but not the antagonist can be increased after PAS. This is strongly dependent on the individualized ISI and on the activation state of the muscle.

Anticipation of elbow joint perturbation shortens the onset time of the reflex EMG response in biceps brachii and triceps brachii

This study aimed to investigate the influence of the anticipation of a perturbation torque applied to extend the elbow joint on the onset time of reflex electromyogram (EMG) responses. A perturbation torque generated by an electromagnetic torque motor system was applied to the forearm of eleven subjects during trials. The trials were divided into an anticipated (AN) condition - perturbation torque applied after the auditory signal - and an unanticipated (UAN) condition - suddenly applied perturbation. To detect the reflex EMG response in the biceps brachii (Bb) and triceps brachii (Tb) muscles, a new method involving the discrete wavelet transform and outlier tests was used. We found that the onset time of the reflex response in both the muscles in the AN condition was significantly shorter than that in the UAN condition. The angle of transition from flexion to extension, which was induced by the reflex response of Bb, was also significantly smaller in the AN condition than in the UAN condition. The results indicate that the anticipation of an applied perturbation torque decreases the onset time of the reflex response in the Bb and Tb.

Reduction of intracortical inhibition in soleus muscle during postural activity

Short-interval intracortical inhibition (SICI) decreases during voluntary contraction of the target muscle. It is unknown whether this effect also occurs with postural contractions. We have compared the effects of voluntary and postural contractions on SICI in the soleus (SOL) muscle. We applied transcranial magnetic stimuli (TMS) in subjects under three tasks: sitting at rest (Rest), sitting while activating the SOL muscle (Voluntary), or standing quietly (Postural). In control trials, we applied suprathreshold TMS to obtain unconditioned motor-evoked potentials (MEPs). In test trials, the same TMS was preceded by a subthreshold TMS at different interstimulus intervals (ISIs), to obtain a conditioned MEP. SICI and intracortical facilitation (ICF) were expressed as the decrease or increase in MEP size relative to unconditioned MEPs. There was significant effect of task in mean SICI or mean ICF in SOL. Mean SICI in SOL was 52% in Rest and decreased to 21% in Voluntary and 15% in Postural. Mean ICF in SOL was 132% and decreased to 113% in Voluntary and to 108% in Postural. Mean SICI in SOL was not different in Voluntary and Postural tasks. There was no effect of task in mean SICI or mean ICF in TA. Our results indicate that decrease of SICI with muscle contraction occurs to a similar extent with tonic voluntary and postural activation, suggesting that those contractions require a similar type of cortical involvement. However, it cannot be excluded that some part of the SICI reduction with muscle contraction depends on changes in segmental excitability.

The effect of task instruction on the excitability of spinal and supraspinal reflex pathways projecting to the biceps muscle

There is controversy within the literature regarding the influence of task instruction on the size of the long-latency stretch reflex (M2) elicited by a joint displacement. The aim of this study was to investigate if the previously reported task-dependent modulation of the M2 is specific to the M2 or can be explained by an early release of the intended voluntary response. We took advantage of the fact that the M2 is absent when the duration of the applied perturbation is less than a critical time period. This allowed us to examine modulation of muscle activity with and without the contribution of the M2. In addition, we applied transcranial magnetic stimulation (TMS) over the primary motor cortex to examine the modulation of corticomotor excitability with task instruction. Elbow joint extension displacements were used to elicit a stretch reflex in the biceps muscle. Subjects were instructed to "do not intervene" (DNI) with the applied perturbation, or to oppose the perturbation by activating the elbow flexors in response to the perturbation (FLEX). Electromyographic (EMG) activity in the time period corresponding to the M2 was significantly facilitated in the FLEX task instruction both with and without the presence of the M2. Motor evoked potentials (MEPs) elicited by TMS were also facilitated during the FLEX condition in the absence of the M2. EMG and MEP responses were not facilitated until immediately prior to the onset of the M2. Paired-pulse TMS revealed a significant reduction in short-interval intracortical inhibition (SICI) during the M2 response, but the level of SICI was not altered by the task instruction. We conclude that the task-dependent modulation of the biceps M2 results, at least in part, from an early release of the prepared movement and is accompanied by an increase in corticospinal excitability that is not specific to the M2 pathway. Task-dependent modulation of the response cannot be explained by an alteration in the excitability of intracortical inhibitory circuits.

Direct corticospinal pathways contribute to neuromuscular control of perturbed stance

The antigravity soleus muscle (Sol) is crucial for compensation of stance perturbation. A corticospinal contribution to the compensatory response of the Sol is under debate. The present study assessed spinal, corticospinal, and cortical excitability at the peaks of short- (SLR), medium- (MLR), and long-latency responses (LLR) after posterior translation of the feet. Transcranial magnetic stimulation (TMS) and peripheral nerve stimulation were individually adjusted so that the peaks of either motor evoked potential (MEP) or H reflex coincided with peaks of SLR, MLR, and LLR, respectively. The influence of specific, presumably direct, corticospinal pathways was investigated by H-reflex conditioning. When TMS was triggered so that the MEP arrived in the Sol at the same time as the peaks of SLR and MLR, EMG remained unaffected. Enhanced EMG was observed when the MEP coincided with the LLR peak ( P < 0.001). Similarly, conditioning of the H reflex by subthreshold TMS facilitated H reflexes only at LLR ( P < 0.001). The earliest facilitation after perturbation occurred after 86 ms. The TMS-induced H-reflex facilitation at LLR suggests that increased cortical excitability contributes to the augmentation of the LLR peaks. This provides evidence that the LLR in the Sol muscle is at least partly transcortical, involving direct corticospinal pathways. Additionally, these results demonstrate that ∼86 ms after perturbation, postural compensatory responses are cortically mediated.

Altered timing of postural reflexes contributes to falling in persons with chronic stroke

The purpose of this study was to determine differences in the timing of postural reflexes and changes in kinematics between those who fell (fallers) in response to standing platform translations and those who did not (non-fallers). Forty-four persons with stroke were exposed to unexpected forward and backward platform translations while standing. Surface electromyography from bilateral tibialis anterior, gastrocnemius, rectus femoris, and biceps femoris were recorded along with kinematic data. Those that fell in response to the translations were compared to those who did not fall in terms of (1) postural reflex onset latency, (2) the time interval between the activation of distal and proximal muscles (i.e. intralimb coupling), and (3) changes in joint angles and trunk motion. Approximately 85% of falls occurred in response to the forward translations. Postural reflex onset latencies were delayed and intralimb coupling durations were longer in the faller versus non-faller group. At the time that the platform completed the translating motion (300 ms), the faller group demonstrated higher trunk velocity, greater change in paretic ankle angle, and the trunk was further behind the ankle compared to the non-faller group. This study suggests that following platform translations, delays in the timing of postural reflexes and disturbed intralimb coupling result in changes in kinematics, which contribute to falls in persons with stroke.

Evidence for a supraspinal contribution to the human quadriceps long-latency stretch reflex

In sitting humans a rapid unexpected lengthening of the knee extensors elicits a stretch reflex (SR) response as recorded by the electromyogram (EMG) which comprises multiple bursts. These are termed short latency responses (SLR), medium latency responses (MLR) and long latency responses (LLR). The aim of this study was to determine if a transcortical pathway contributes to any of these bursts. Flexion perturbations (amplitude =4 degrees, velocity=150 degrees/s) were imposed on the right knee joint of sitting subjects (n=11). The effect of the perturbation on the electromyographic (EMG) response of the pre-contracted quadriceps muscle to magnetic stimulation of the contralateral motor cortex was quantified. Transcranial magnetic stimulation (TMS) was applied to elicit a compound motor evoked potential (MEP) in the target muscle rectus femoris (RF), in the vastus lateralis (VL), vastus medialis (VM) and biceps femoris (BF). The MEP and SR were elicited either in combination or separately. When applied in combination the delay between the SR and the MEP varied from 0 to 150 ms in steps of 4, 5 and 10 ms. Somatosensory evoked potentials (SEPs) were recorded from four subjects during the imposed stretch to quantify the latency of the resulting afferent volley. Onset latencies of responses in RF were 25+/-2 ms for the SR and 20+/-4 ms for the MEP. The average SEP latency was 24+/-2 ms. A transcortical pathway thus has the potential to contribute to the RF SR no earlier than 54+/-6 ms (SEP + MEP + 10 ms central processing delay) following the stretch onset. The duration of the total reflex burst was 85+/-6 ms. Significant facilitation of the MEP commenced at 78 ms, coinciding with the LLR component of the stretch response. No such facilitation was observed in the synergists VL and VM, or in the antagonist BF. Our results indicate that the LLR of the RF likely involves supraspinal pathways. More importantly, of the investigated muscles, this involvement of higher centers in the shaping of the LLR is specific to the RF muscle during the investigated task.

Reflex contribution of spindle group Ia and II afferent input to leg muscle spasticity as revealed by tendon vibration in hemiparesis

OBJECTIVE

Foot dorsiflexion evokes a short- (SLR) and a medium-latency EMG response (MLR) in the soleus of standing subjects. SLR is mediated by spindle group Ia, while group II fibres contribute to MLR through an oligosynaptic circuit. We studied the effects of Achilles' tendon vibration on both responses in spastic patients to disclose any abnormal excitability of these pathways.

METHODS

SLR and MLR were evoked in 11 hemiparetics and 11 normals. The vibration-induced changes in both responses were correlated to the Ashworth score of the affected leg.

RESULTS

There were no differences between normals and patients in the size of control SLR or MLR. Vibration decreased SLR to 70% in normal subjects, but increased it to 110% in patients, in both affected and unaffected leg. Vibration did not affect MLR in normals, but increased it to 165% on the affected and 120% on the unaffected side of patients. Ashworth score was solely correlated with the degree of vibration-induced increase of MLR.

CONCLUSIONS

While the lack of inhibitory effect of vibration on SLR confirms a reduced inhibitibility of the monosynaptic reflex, the increased MLR indicates a disinhibition of group II pathway in patients, connected to the loss of descending control on group II interneurones. Spastic hypertonia depends on release of group II rather than group Ia reflex pathways.

SIGNIFICANCE

These findings give a neurophysiological support for the pharmacological treatment of spastic hypertonia and suggest a method for the assessment of its effects.

Intact ability to learn internal models of arm dynamics in Huntington's disease but not cerebellar degeneration

Two different compensatory mechanisms are engaged when the nervous system senses errors during a reaching movement. First, on-line feedback control mechanisms produce in-flight corrections to reduce errors in the on-going movement. Second, these errors modify the internal model with which the motor plan is transformed into motor commands for the subsequent movements. What are the neural mechanisms of these compensatory systems? In a previous study, we reported that while on-line error correction was disturbed in patients with Huntington's disease (HD), it was largely intact in patients with cerebellar degeneration. Here we altered dynamics of reaching and studied the effect of error in one trial on the motor commands that initiated the subsequent trial. We observed that in patients with cerebellar degeneration, motor commands changed from trial-to-trial by an amount that was comparable to control subjects. However, these changes were random and were uninformed by the error in the preceding trial. In contrast, the change in motor commands of HD patients was strongly related to the error in the preceding trial. This error-dependent change had a sensitivity that was comparable to healthy controls. As a result, HD patients exhibited no significant deficits in adapting to novel arm dynamics, whereas cerebellar subjects were profoundly impaired. These results demonstrate a double dissociation between on-line and trial-to-trial error correction suggesting that these compensatory mechanisms have distinct neural bases that can be differentially affected by disease.

Effects of arousal and natural baroreceptor activation on the human muscle stretch reflex

The nociceptive flexion reflex is inhibited during systole; this inhibition may be due to increased baroreceptor stimulation. It is yet to be determined whether other spinal reflexes are similarly modulated across the cardiac cycle. There is also evidence that stretch and tendon reflexes are facilitated by increased arousal. This study investigated the effects of phase of the cardiac cycle and arousal on the muscle stretch reflex components M1, M2, and M3. Stretch reflexes were elicited in leg muscles at six intervals across the cardiac cycle during rest, number repetition, and mental arithmetic. Mental arithmetic provoked increased cardiovascular arousal and facilitated both M1 and M2 compared to rest and number repetition. The stretch reflex did not vary with the phase of the cardiac cycle. While the stretch reflex is susceptible to arousal, natural baroreceptor-mediated modulation across the cardiac cycle may be specific to nociception.

Neural coupling between upper and lower limbs during recumbent stepping

During gait rehabilitation, therapists or robotic devices often supply physical assistance to a patient's lower limbs to aid stepping. The expensive equipment and intensive manual labor required for these therapies limit their availability to patients. One alternative solution is to design devices where patients could use their upper limbs to provide physical assistance to their lower limbs (i.e., self-assistance). To explore potential neural effects of coupling upper and lower limbs, we investigated neuromuscular recruitment during self-driven and externally driven lower limb motion. Healthy subjects exercised on a recumbent stepper using different combinations of upper and lower limb exertions. The recumbent stepper mechanically coupled the upper and lower limbs, allowing users to drive the stepping motion with upper and/or lower limbs. We instructed subjects to step with 1) active upper and lower limbs at an easy resistance level (active arms and legs); 2) active upper limbs and relaxed lower limbs at easy, medium, and hard resistance levels (self-driven); and 3) relaxed upper and lower limbs while another person drove the stepping motion (externally driven). We recorded surface electromyography (EMG) from six lower limb muscles. Self-driven EMG amplitudes were always higher than externally driven EMG amplitudes ( P < 0.05). As resistance and upper limb exertion increased, self-driven EMG amplitudes also increased. EMG bursts during self-driven and active arms and legs stepping occurred at similar times. These results indicate that active upper limb movement increases neuromuscular activation of the lower limbs during cyclic stepping motions. Neurologically impaired humans that actively engage their upper limbs during gait rehabilitation may increase neuromuscular activation and enhance activity-dependent plasticity.

The effect of transcranial magnetic stimulation and peripheral nerve stimulation on corticomuscular coherence in humans

Cortex and muscle show coupled oscillations in the 15-35 Hz frequency band during voluntary movements. To obtain evidence of the neuronal network responsible for this rhythmicity we investigated the effect of transcranial magnetic stimulation (TMS) and peripheral nerve stimulation on the coupling between eletcroencephalographic (EEG) activity recorded from the scalp over the motor cortex and electromyographic (EMG) activity recorded from the tibialis anterior (TA) muscle in 15 healthy human subjects. TMS over the leg area at intensities between 0.95 and 1.1 x threshold for a motor evoked potential (MEP) in the TA increased corticomuscular coherence in the 15-35 Hz frequency band. This effect lasted on average for 300 ms, but could last up to 600-800 ms in some subjects. Stimulation of motor nerves from the ankle muscles suppressed corticomuscular coherence in the 15-35 Hz frequency range between leg area EEG and TA EMG for a period up to 600-800 ms. In addition, increased coherence around 10 Hz was observed for a period up to 250 ms after the stimulation. Stimulation of motor nerves in the arm and motor nerves from the ankle muscles in the other leg had no effect. The findings indicate that TMS has direct access to the neuronal circuitry in the motor cortex, which generates the corticomuscular coherence. This effect was caused either by direct activation of corticospinal cells or by activation of local neuronal circuitries in the motor cortex. The effects of peripheral nerve stimulation suggest that an alternative rhythm generator may entrain the cortical cells into a lower 10 Hz rhythm and disrupt the 15-35 Hz rhythm.

The time course for kinetic versus kinematic planning of goal-directed human motor behavior

The present psychophysical study compares motor planning during goal-directed reaching movements and isometric spatial force generation. Our objective is to characterize the extent to which the motor system accounts for the biomechanical details of an impending reach. One issue that the nervous system must take into account when transforming a spatial sensory signal into an intrinsic pattern of joint torques is that of limb dynamics, including intersegmental dynamics and inertial anisotropy of the arm. These will act to displace the hand away from a straight path to an object. In theory, if the nervous system accounts for movement-related limb dynamics prior to its initial motor output, early force direction for a movement will differ from an isometric force to the same spatial target. Alternatively, biomechanical details of motor behavior may be implemented into the motor act following its initiation. Limb position and force output at the wrist were recorded while subjects displaced a cursor to targets viewed on a computer monitor. To generate isometric forces, a magnetic brake held a mechanical linkage supporting the arm in place. Subjects were cued to displace the cursor by using either isometric force or limb movement. On random trials, a movement was cued but an isometric force was unexpectedly required. Results show that there is not a significant directional difference in the initial force trajectory when planning a movement versus planning an isometric force. These findings suggest that the motor system may initially use a coarse approximation of movement-related limb dynamics, allowing for the refinement of the motor plan as the movement unfolds.

Characterisation of the quadriceps stretch reflex during the transition from swing to stance phase of human walking

The main objective of this study was to characterize the stretch reflex response of the human thigh muscles to an unexpected knee flexion at the transition from stance to swing during walking. Eleven healthy subjects walked on a treadmill at their preferred speed. Reliable and constant knee flexions (6-12 degrees amplitude, 230-350 degrees /s velocity, 220 ms duration) were applied during the late swing and early stance phase of human walking by rotating the knee joint with a specifically designed portable stretch apparatus affixed to the left knee. Responses from rectus femoris (RF), vastus lateralis (VL), vastus medialis (VM), biceps femoris (BF), medial hamstrings (MH) and medial gastrocnemius (GM) were recorded via bipolar surface electromyograms (EMG). The onset of the response in the RF, VL and VM, remained stable and independent of the time in the step cycle when the stretch was applied. Across all subjects the response onset (mean +/- SD) occurred at 23+/-1, 24+/-1 and 23+/-1 ms for RF, VL and VM, respectively. The duration of the initial response was 90-110 ms, at which time the EMG signal returned towards baseline levels. Three reflex response windows, labelled the short latency reflex (SLR), the medium latency reflex (MLR) and the late latency reflex response (LLR), were analysed. The medium and late reflex responses of all knee extensors increased significantly ( p=0.008) as the gait cycle progressed from swing to stance. This was not related to the background EMG activity. In contrast, during standing at extensor EMG levels similar to those attained during walking the reflex responses were dependent on background EMG. During walking, LLR amplitudes expressed as a function of the background activity were on average two to three times greater than SLR and MLR reflex amplitudes. Distinct differences in SLR and LLR amplitude were observed for RF, VL and VM but not in the MLR amplitude. This may be related to the different pathways mediating the SLR, MLR and LLR components of the stretch response. As for the knee extensor antagonists, they exhibited a response to the stretch of the quadriceps at latencies short enough to be monosynaptic. This is in agreement with the suggestion by Eccles and Lundberg (1958) that there may be functional excitatory connections between the knee extensors and flexors in mammals.

Stretch reflex excitability of the anti-gravity ankle extensor muscle in elderly humans

AIM

To examine whether the stretch reflex excitability of the soleus muscle changes with age, stretch reflexes at rest (REST) and during weak voluntary contractions (ACT) were elicited in 18 older and 14 younger subjects.

METHOD

The amplitude of the stretch reflex responses and gain, defined as the gradient of the regression line for the relation between stretch reflex responses against the angular velocity of the applied perturbation, were evaluated in each short-latency (M1) and two long-latency components (M2 and M3).

RESULTS

It was found that in the older group, both the amplitude and gain of the M1 component did not change from the REST to the ACT conditions, whereas in the younger group both variables significantly increased from the REST to ACT conditions. The latency of the M1 component was significantly shorter under the REST condition (older vs. younger: 51.8 +/- 7.37 vs. 55.1 +/- 8.69 ms), while no group differences were found in those variables under the ACT condition, suggesting that the muscle-tendon complexes of SOL muscles of the older subjects were less elastic and had less slack, probably due to age-related histochemical alterations. Further, the Hoffman reflex (H-reflex), elicited during the REST condition in 10 older and 11 younger subjects showed no significant differences, suggesting that the soleus motoneuron response to the Ia input was comparable between the two subject groups.

CONCLUSION

The histochemical alterations occurring with the ageing process might augment the short-latency stretch reflex in the SOL muscle without enhancement of motoneuronal excitability, and this effect might be masked when the muscle is voluntarily activated.

Investigating human motor control by transcranial magnetic stimulation

In this review we discuss the contribution of transcranial magnetic stimulation (TMS) to the understanding of human motor control. Compound motor-evoked potentials (MEPs) may provide valuable information about corticospinal transmission, especially in patients with neurological disorders, but generally do not allow conclusions regarding the details of corticospinal function to be made. Techniques such as poststimulus time histograms (PSTHs) of the discharge of single, voluntarily activated motor units and conditioning of H reflexes provide a more optimal way of evaluating transmission in specific excitatory and inhibitory pathways. Through application of such techniques, several important issues have been clarified. TMS has provided the first real evidence that direct monosynaptic connections from the motor cortex to spinal motoneurons exist in man, and it has been revealed that the distribution of these projections roughly follows the same proximal-distal gradient as in other primates. However, pronounced differences also exist. In particular, the tibialis anterior muscle appears to receive as significant a monosynaptic corticospinal drive as muscles in the hand. The reason for this may be the importance of this muscle in controlling the foot trajectory in the swing phase of walking. Conditioning of H reflexes by TMS has provided evidence of changes in cortical excitability prior to and during various movements. These experiments have generally confirmed information obtained from chronic recording of the activity of corticospinal cells in primates, but information about the corticospinal contribution to movements for which information from other primates is sparse or lacking has also been obtained. One example is walking, where TMS experiments have revealed that the corticospinal tract makes an important contribution to the ongoing EMG activity during treadmill walking. TMS experiments have also documented the convergence of descending corticospinal projections and peripheral afferents on spinal interneurons. Current investigations of the functional significance of this convergence also rely on TMS experiments. The general conclusion from this review is that TMS is a powerful technique in the analysis of motor control, but that care is necessary when interpreting the data. Combining TMS with other techniques such as PSTH and H reflex testing amplifies greatly the power of the technique.

Reflex responses in the lower leg following landing impact on an inverting and non-inverting platform

In the lower leg, landing after a jump induces reflexes, the role of which is not well understood. This is even more so for reflexes following landing on inverting surfaces. The latter condition is of special interest since ankle inversion traumata are one of the most common injuries during sport. Most studies have investigated ankle inversions during a static standing condition. However, ankle injuries occur during more dynamic activities such as jumping. Therefore, the present study aimed at reproducing these situations but in a completely safe setting. EMG responses were recorded after landing on an inverting surface, which caused a mild ankle inversion of 25 deg of rotation (in a range sufficient to elicit reflexes but safe enough to exclude sprains). The results are compared with data from landing on a non-inverting surface to understand the effect of the inversion. In general, landing on the platform resulted in short and long latency responses (SLR and LLR) in triceps surae (soleus, gastrocnemius medialis and lateralis) and peroneal muscles (long and short peroneal) but not in the tibialis anterior muscle. Landing on the inverting platform caused significant LLRs in the peroneal muscles (which underwent the largest stretch) but not in the triceps muscles. Conversely, landing on a non-inverting platform induced larger SLRs in triceps than in the peroneal muscles. Although the peroneal LLRs thus appeared to be selectively recruited in an inverting perturbation, their role during such perturbations should be limited since the latency of these responses was about 90 ms while the inversion lasts only 42 ms. The SLRs, if present, had an onset latency of around 44 ms. In the period following the inversion, however, the responses may be important in preventing further stretch of these muscles.

Facilitation of both stretch reflex and corticospinal pathways of the tibialis anterior muscle during standing in humans

Excitability of both stretch reflex (SR) and motor evoked potential (MEP) elicited in the tibialis anterior (TA) muscle by transcranial magnetic stimulation were tested in standing humans. The results demonstrated significantly greater values for both SR and MEP in the TA while standing than while in the supine posture, although background electromyographic activity was silent in the two conditions. Taken together with previous reports that both pathways are facilitated in the TA at the early stance phase of human walking, our findings suggest that a common neural mechanism underlies both observations, one that might be functionally relevant for securing ankle joint stabilization during upright standing.

Modulation of afferent inflow during the control of balancing tasks using the lower limbs

This study investigated the control of sensory inputs during the performance of an inverted-pendulum balancing task. Experiments were conducted to examine modulation of proprioceptive inputs during balance tasks of varying difficulty. It was hypothesized that proprioceptive inputs to both spinal and cortical levels would be facilitated during a challenged balance task. In contrast, during challenged balance control, results revealed task-specific facilitation of sensory inputs to the cortex and inhibition of the spinal reflex pathway. Observations of increased transmission of proprioceptive inputs to the cortex and decreased transmission at the spinal level suggest that the cortex plays an important role in challenged balance, whereas the role for the spinal stretch reflex appears to be less important.

A real-time state predictor in motor control: study of saccadic eye movements during unseen reaching movements

Theoretical motor control predicts that because of delays in sensorimotor pathways, a neural system should exist in the brain that uses efferent copy of commands to the arm, sensory feedback, and an internal model of the dynamics of the arm to predict the future state of the hand (i.e., a forward model). We tested this theory under the hypothesis that saccadic eye movements, tracking an unseen reaching movement, would reflect the output of this state predictor. We found that in unperturbed reaching movements, saccade occurrence at any time t consistently provided an unbiased estimate of hand position at t + 196 msec. To investigate the behavior of this predictor during feedback error control, we applied 50 msec random-force perturbations to the moving hand. Saccades showed a sharp inhibition at 100 msec after perturbation. At approximately 170 msec, there was a sharp increase in saccade probabilities. These postperturbation saccades were an unbiased estimator of hand position at saccade time t + 150 msec. The ability of the brain to guide saccades to the future position of the hand failed when a force field unexpectedly changed the dynamics of the hand immediately after perturbation. The behavior of the eyes suggested that during reaching movements, the brain computes an estimate of future hand position based on an internal model that relies on real-time proprioceptive feedback. When an error occurs in reaching movements, the estimate of future hand position is recomputed. The saccade inhibition period that follows the hand perturbation may indicate the length of time it takes for this computation to take place.

A real-time state predictor in motor control: study of saccadic eye movements during unseen reaching movements

Theoretical motor control predicts that because of delays in sensorimotor pathways, a neural system should exist in the brain that uses efferent copy of commands to the arm, sensory feedback, and an internal model of the dynamics of the arm to predict the future state of the hand (i.e., a forward model). We tested this theory under the hypothesis that saccadic eye movements, tracking an unseen reaching movement, would reflect the output of this state predictor. We found that in unperturbed reaching movements, saccade occurrence at any time t consistently provided an unbiased estimate of hand position at t + 196 msec. To investigate the behavior of this predictor during feedback error control, we applied 50 msec random-force perturbations to the moving hand. Saccades showed a sharp inhibition at 100 msec after perturbation. At approximately 170 msec, there was a sharp increase in saccade probabilities. These postperturbation saccades were an unbiased estimator of hand position at saccade time t + 150 msec. The ability of the brain to guide saccades to the future position of the hand failed when a force field unexpectedly changed the dynamics of the hand immediately after perturbation. The behavior of the eyes suggested that during reaching movements, the brain computes an estimate of future hand position based on an internal model that relies on real-time proprioceptive feedback. When an error occurs in reaching movements, the estimate of future hand position is recomputed. The saccade inhibition period that follows the hand perturbation may indicate the length of time it takes for this computation to take place.

A cumulative sum test for a peri-stimulus time histogram using the Monte Carlo method

We have established a cumulative sum (CUSUM) test for a peri-stimulus time histogram (PSTH) for the case where a conditioning stimulus is delivered at a fixed interval after previous discharge of a motor unit. (We refer to this kind of PSTH as an 'arranged PSTH'). Expectations of the firing probability after the conditioning stimulus vary among the bins in this arranged PSTH, while the expectations among the bins are all the same in the original PSTH; thus, we could not apply conventional tests for statistical analysis. We, therefore, propose a novel CUSUM test that uses the Monte Carlo method. With this method, the range of the statistical scattering noise on a CUSUM is computationally found by simulating the statistical process in order to calculate the confidence interval. We verified this CUSUM test using both simulated and actual experiments. This paper presents the procedure for performing this new method, along with an example of its application.

Statistical test for peri-stimulus time histograms in assessing motor neuron activity

The peri-stimulus time histogram is a valuable tool for evaluating neural connections in humans. To detect the degree to which a conditioning stimulus to a sensory nerve modulates motor neuron activity, a histogram of motor unit spike intervals after a conditioning stimulus is measured. This histogram allows the effect of the conditioning stimulus to be visualised. By comparison with a reference histogram of motor unit spike intervals after a sham stimulus, the noise caused by spontaneous firing sway can be removed. However, no valid statistical test has yet been developed to separate the physiological effect from the spontaneous sway and statistical noise. A computational method has been proposed to detect modulation caused by a conditioning stimulus. To clarify the effect of a conditioning stimulus, this new method used reference histograms to calculate a confidence interval. A simulated experiment demonstrated that about 2000 re-samplings were sufficient to estimate a confidence interval for a histogram with 1 ms bin width constructed from 300 triggers. Testing of the experimental data, measured from the tibialis anterior muscles during the elicitation of the excitatory spinal reflex, confirmed that significant peaks were produced at 30, 34, 35 and 38ms after the conditioning stimulus. These correspond appropriately to the delay of the spinal reflex.

Learning the dynamics of reaching movements results in the modification of arm impedance and long-latency perturbation responses

Some characteristics of arm movements that humans exhibit during learning the dynamics of reaching are consistent with a theoretical framework where training results in motor commands that are gradually modified to predict and compensate for novel forces that may act on the hand. As a first approximation, the motor control system behaves as an adapting controller that learns an internal model of the dynamics of the task. It approximates inverse dynamics and predicts motor commands that are appropriate for a desired limb trajectory. However, we had previously noted that subtle motion characteristics observed during changes in task dynamics challenged this simple model and raised the possibility that adaptation also involved sensory-motor feedback pathways. These pathways reacted to sensory feedback during the course of the movement. Here we hypothesize that adaptation to dynamics might also involve a modification of how the CNS responds to sensory feedback. We tested this through experiments that quantified how the motor system's response to errors during voluntary movements changed as it adapted to dynamics of a force field. We describe a nonlinear approach that approximates the impedance of the arm, i.e., force response as a function of arm displacement trajectory. We observe that after adaptation, the impedance function changes in a way that closely matches and counters the effect of the force field. This is particularly prominent in the long-latency (> 100 ms) component of response to perturbations. Therefore, it appears that practice not only modifies the internal model with which the brain generates motor commands that initiate a movement, but also the internal model with which sensory feedback is integrated with the ongoing descending commands in order to respond to error during the movement.

Transcranial magnetic stimulation and stretch reflexes in the tibialis anterior muscle during human walking

Stretch of the ankle dorsiflexors was applied at different times of the walking cycle in 17 human subjects. When the stretch was applied in the swing phase, only small and variable reflex responses were observed in the active tibialis anterior (TA) muscle. Two of the reflex responses that could be distinguished had latencies which were comparable with the early (M1) and late (M3)components of the three reflex responses (M1, M2 and M3) observed during tonic dorsiflexion in sitting subjects. In the stance phase a single very large response was consistently observed in the inactive TA muscle. The peak of this response had the same latency as the peak of M3, but in the majority of subjects the onset latency was shorter than that of M3. The TA reflex response in the stance phase was abolished by ischaemia of the lower leg at the same time as the soleus H-reflex, suggesting that large muscle afferents were involved in the generation of the response. Motor-evoked potentials (MEPs) elicited in the TA by transcranial magnetic stimulation (TMS) were strongly facilitated corresponding to the peak of the stretch response in the stance phase and the late reflex response in the swing phase. A similar facilitation was not observed corresponding to the earlier responses in the swing phase and the initial part of the response in stance. Prior stretch did not facilitate MEPs evoked by transcranial electrical stimulation in the swing phase of walking. However, in the stance phase MEPs elicited by strong electrical stimulation were facilitated by prior stretch to the same extent as the MEPs evoked by TMS. The large responses to stretch seen in the stance phase are consistent with the idea that stretch reflexes are mainly involved in securing the stability of the supporting leg during walking. It is suggested that a transcortical reflex pathway may be partly involved in the generation of the TA stretch responses during walking.

Evidence for transcortical reflex pathways in the lower limb of man

The existence of transcortical reflex pathways in the control of distal arm and hand muscles in man is now widely accepted. Much more controversy exists regarding a possible contribution of such reflexes to the control of leg muscles. It is often assumed that transcortical reflex pathways play no, or only a minor, role in the control of leg muscles. Transcortical reflex pathways according to this view are reserved for the control of the distal upper limb and are seen in close relation to the evolution of the primate hand. Here we review data, which provide evidence that transcortical reflexes do exist for lower limb muscles and may play a significant role in the control of at least some of these muscles. This evidence is based on animal research, recent experiments combining transcranial magnetic stimulation with peripheral electrical and mechanical stimulation in healthy subjects and neurological patients. We propose that afferent activity from muscle and skin may play a role in the regulation of bipedal gait through transcortical pathways.

Differential changes in corticospinal and Ia input to tibialis anterior and soleus motor neurones during voluntary contraction in man

Motor-evoked potentials (MEPs) were recorded in the tibialis anterior and soleus muscles following transcranial magnetic stimulation (TMS) of the motor cortex. In the soleus, the H-reflex amplitude increased with the contraction level to the same extent as that of MEPs, whereas in the tibialis anterior, the H-reflex amplitude increased significantly less than that of MEPs. The latency of the MEPs decreased with contraction, whereas this was not the case of the H-reflexes. In the tibialis anterior, the response probability of single-motor units (SMU) to TMS increased more substantially during voluntary contraction than following stimulation of the peroneal nerve. In the tibialis anterior, the response probability of SMU increased more substantially during voluntary contraction than following stimulation of the peroneal nerve. The short-latency facilitation, presumably monosynaptic of origin, of the soleus H-reflex evoked by subthreshold TMS increased as a function of the plantarflexion force. This was not the case for the heteronymous Ia facilitation of the soleus H-reflex following stimulation of the femoral nerve. It is concluded that the corticospinal input to lower limb motor neurones generated by TMS increases with the level of voluntary contraction, whereas this is true only to a limited extent for the synaptic input from Ia afferents. It is suggested that this reflects changes in the susceptibility of corticospinal cells to TMS during voluntary contraction.

Long-latency component of the stretch reflex in human muscle is not mediated by intramuscular stretch receptors

Reflex responses to mechanical stimulation of muscle (brief imposed movement) were investigated. Reflexes were elicited in the forefinger, recording from the first dorsal interosseous (FDI), and in the foot, recording from soleus. These responses typically consisted of a short-latency component (M1) and a long-latency component (M2) at 33 ms and 53 ms, respectively, after the stimulus in the case of FDI, and 37 ms and 68 ms, respectively, in soleus upon stimulation of the sole of the foot. Normally, when a muscle is stretched by a mechanical stimulus (either naturally or by an experimentally imposed movement), both skin receptors and muscle stretch receptors are activated. It is possible, however, to devise stimulation parameters where this is not the case. Fixating the finger with plasticine enables the effects of skin stimulation to be studied without stretching the FDI muscle. On the other hand, tapping a long tendon allows muscle stretch receptors to be activated without involving skin or subcutaneous structures. Component M1 was always abolished by finger fixation in 40 trials on 10 subjects, with M2 being essentially unchanged in latency, duration, or amplitude. Reflex responses were obtained in soleus muscle in nine experiments by prodding the sole of the foot (thereby stimulating both skin and muscle stretch receptors). Alternatively, the tendo achilles was prodded (which solely activates stretch receptors in the muscle). In the former, M1 and M2 were generated. In the latter, only M1 was produced. It is concluded that the long-latency component of the stretch reflex, M2, originates in skin and/or subcutaneous nerve terminals and that no part of M2 originates in muscle stretch receptors.

Muscular responses and movement strategies during stumbling over obstacles

Although many studies have investigated reflexes after stimulation of either cutaneous or proprioceptive afferents, much less is known about responses after more natural perturbations, such as stumbling over an obstacle. In particular, the phase dependency of these responses and their relation to the stumbling behavior has received little attention. Hence response strategies during stumbling reactions after perturbations at different times in the swing phase of gait were studied. While subjects walked on a treadmill, a rigid obstacle unexpectedly obstructed the forward sway of the foot. All subjects showed an "elevating strategy" after early swing perturbations and a "lowering strategy" after late swing perturbations. During the elevating strategy, the foot was directly lifted over the obstacle through extra knee flexion assisted by ipsilateral biceps femoris (iBF) responses and ankle dorsiflexion assisted by tibialis anterior (iTA) responses. Later, large rectus femoris (iRF) activations induced knee extension to place the foot on the treadmill. During the lowering strategy, the foot was quickly placed on the treadmill and was lifted over the obstacle in the subsequent swing. Foot placement was actively controlled by iRF and iBF responses related to knee extension and deceleration of the forward sway. Activations of iTA mostly preceded the main ipsilateral soleus (iSO) responses. For both strategies, four response peaks could be distinguished with latencies of approximately 40 ms (RP1), approximately 75 ms (RP2), approximately 110 ms (RP3), and approximately 160 ms (RP4). The amplitudes of these response peaks depended on the phase in the step cycle. The phase-dependent modulation of the responses could not be accounted for by differences in stimulation or in background activity and therefore is assumed to be premotoneuronal in origin. In mid swing, both the elevating and lowering strategy could occur. For this phase, the responses of the two strategies could be compared in the absence of phase-dependent response modulation. Both strategies had the same initial electromyographic responses till approximately 100 ms (RP1-RP2) after perturbation. The earliest response (RP1) is assumed to be a short-latency stretch reflex evoked by the considerable impact of the collision, whereas the second (RP2) has features reminiscent of cutaneous and proprioceptive responses. Both these responses did not determine the behavioral response strategy. The functionally important response strategies depended on later responses (RP3-RP4). These data suggest that during stumbling reactions, as a first line of defense, the CNS releases a relatively aspecific response, which is followed by an appropriate behavioral response to avoid the obstacle.

Major role for sensory feedback in soleus EMG activity in the stance phase of walking in man

1. Sensory feedback plays a major role in the regulation of the spinal neural locomotor circuitry in cats. The present study investigated whether sensory feedback also plays an important role during walking in 20 healthy human subjects, by arresting or unloading the ankle extensors 6 deg for 210 ms in the stance phase of gait. 2. During the stance phase of walking, unloading of the ankle extensors significantly (P < 0.05) reduced the soleus activity by 50 % in early and mid-stance at an average onset latency of 64 ms. 3. The onset and amplitude of the decrease in soleus activity produced by the unloading were unchanged when the common peroneal nerve, which innervates the ankle dorsiflexors, was reversibly blocked by local injection of lidocaine (n = 3). This demonstrated that the effect could not be caused by a peripherally mediated reciprocal inhibition from afferents in the antagonist nerves. 4. The onset and amplitude of the decrease in soleus activity produced by the unloading were also unchanged when ischaemia was induced in the leg by inflating a cuff placed around the thigh. At the same time, the group Ia-mediated short latency stretch reflex was completely abolished. This demonstrated that group Ia afferents were probably not responsible for the decrease of soleus activity produced by the unloading. 5. The findings demonstrate that afferent feedback from ankle extensors is of significant importance for the activation of these muscles in the stance phase of human walking. Group II and/or group Ib afferents are suggested to constitute an important part of this sensory feedback.

Soleus long-latency stretch reflexes during walking in healthy and spastic humans

The present study was carried out to investigate the long-latency soleus stretch reflexes M2 (peak latency of approximately 85 ms) and M3 (peak latency of approximately 115 ms) during walking in healthy and spastic multiple sclerosis (MS) patients. An 8 degrees stretch was applied to the ankle extensors of the left leg in 8 healthy subjects during normal walking speed and 9 spastic MS patients and 10 age-matched healthy subjects during slow walking. When present in walking healthy subjects, M2 and M3 were modulated in a similar way and with the same amplitudes as previously described for the short latency soleus stretch reflex (M1). The spastic patients' soleus M1 was significantly less modulated during walking. The patients' M2 long-latency response was modulated in the same way as the age-matched healthy subjects. All patients' M3 responses were absent or much suppressed during walking. The origin and functional importance of the short- and long-latency stretch reflexes in healthy and spastic persons are discussed in relation to the above findings and the behaviour of the stretch reflexes during matched isometric contractions. M3 is argued to be part of a transcortical reflex in healthy subjects.

The effect of transcranial magnetic stimulation on the soleus H reflex during human walking

1. The effect of transcranial magnetic stimulation (TMS) on the soleus H reflex was investigated in the stance phase of walking in seventeen human subjects. For comparison, measurements were also made during quiet standing, matched tonic plantar flexion and matched dynamic plantar flexion. 2. During walking and dynamic plantar flexion subliminal (0.95 times threshold for a motor response in the soleus muscle) TMS evoked a large short-latency facilitation (onset at conditioning-test interval: -5 to -1 ms) of the H reflex followed by a later (onset at conditioning-test interval: 3-16 ms) long-lasting inhibition. In contrast, during standing and tonic plantar flexion the short-latency facilitation was either absent or small and the late inhibition was replaced by a long-lasting facilitation. 3. When grading the intensity of TMS it was found that the short-latency facilitation had a lower threshold during walking than during standing and tonic plantar flexion. Regardless of the stimulus intensity the late facilitation was never seen during walking and dynamic plantar flexion and the late inhibition was not seen, except for one subject, during standing and tonic plantar flexion. 4. A similar difference in the threshold of the short-latency facilitation between walking and standing was not observed when the magnetic stimulation was replaced by transcranial electrical stimulation. 5. The lower threshold of the short-latency facilitation evoked by magnetic but not electrical transcranial stimulation during walking compared with standing suggests that cortical cells with direct motoneuronal connections increase their excitability in relation to human walking. The significance of the differences in the late facilitatory and inhibitory effects during the different tasks is unclear.